Layer half of the sauerkraut in the bottom of the pot. Pour 1/3 cup dressing over it; place 3 chicken breasts on top and spread the mustard on chicken. Top with the remaining sauerkraut and chicken breasts; pour another 1/3 cup of dressing over all and reserve the remaining 1/3 cup of dressing for serving.

Cover and cook on LOW for about 4 hours, or until chicken is cooked through and tender. Sprinkle Swiss cheese and cook until cheese is melted. Serve with reserved dressing.